Naismith and Wooden are both very full and very tough at the D3 level. Rupp is pretty tough too, I'd say. But none of them are either slam-full or embarrassingly empty. And you can have fun in a relatively empty world as long as you're in a good conference.
For someone new, I'd say the best balance is to find a job in a mostly full but geographically spread out conference. For instance, my first job was Maryville in the Ohio Conference in Crum. There were 6-7 humans in that conference (and are now even more), but Maryville was far enough away from the other ones that I really didn't butt heads in recruiting too often. The University Conference is often a good choice for this, if it has enough people in it. It's relatively spread out. But it really does depend on where the humans are and where they aren't. I can't tell you where all the best spots in every world are. The only D3 conference I'm in with beginner-friendly openings is probably the USA South in Smith, but you'd have to wait until May for that one to roll over.
But if you're the sort of person who really likes doing this kind of research for yourself instead of just picking a job in a decently full conference, you can check out
the map of D3 schools and cross-reference with the job openings to see which conferences are more full and which are less full. The ideal, to make it fun and make recruiting a bit easier, is to be in a more full conference but with a bunch of relatively empty conferences in your immediate geographic vicinity.